The Basics

class

Basic class definitions begin with the
keyword class, followed by a class name,
followed by a pair of curly braces which enclose the definitions
of the properties and methods belonging to the class.

The class name can be any valid label, provided it is not a
PHP reserved word. A valid class
name starts with a letter or underscore, followed by any number of
letters, numbers, or underscores. As a regular expression, it
would be expressed thus:
^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$.

A class may contain its
own constants, variables
(called "properties"), and functions (called "methods").

The pseudo-variable $this is available when a
method is called from within an object
context. $this is a reference to the calling
object (usually the object to which the method belongs, but
possibly another object, if the method is called
statically from the context
of a secondary object).
As of PHP 7.0.0 calling a non-static method statically from an incompatible
context results in $this being undefined inside the method. Calling a
non-static method statically from an incompatible context has been
deprecated as of PHP 5.6.0. As of PHP 7.0.0 calling a non-static method
statically has been generally deprecated (even if called from a compatible
context). Before PHP 5.6.0 such calls already triggered a strict notice.

Example #2 Some examples of the $this pseudo-variable

We're assuming that error_reporting is disabled for this example;
otherwise the following code would trigger deprecated and strict notices,
respectively, depending on the PHP version.

$this is defined (A)
$this is not defined.
$this is defined (B)
$this is not defined.

Output of the above example in PHP 7:

$this is defined (A)
$this is not defined.
$this is not defined.
$this is not defined.

new

To create an instance of a class, the new keyword must
be used. An object will always be created unless the object has a
constructor defined that throws an
exception on error. Classes
should be defined before instantiation (and in some cases this is a
requirement).

If a string containing the name of a class is used with
new, a new instance of that class will be created. If
the class is in a namespace, its fully qualified name must be used when
doing this.

Example #3 Creating an instance

<?php$instance = new SimpleClass();

// This can also be done with a variable:$className = 'SimpleClass';$instance = new $className(); // new SimpleClass()?>

In the class context, it is possible to create a new object by
new self and new parent.

When assigning an already created instance of a class to a new variable, the new variable
will access the same instance as the object that was assigned. This
behaviour is the same when passing instances to a function. A copy
of an already created object can be made by
cloning it.

PHP 5.4.0 introduced the possibility to access a member of a newly created
object in a single expression:

Example #6 Access member of newly created object

<?phpecho (new DateTime())->format('Y');?>

The above example will output
something similar to:

2016

Properties and methods

Class properties and methods live in separate "namespaces", so it is
possible to have a property and a method with the same name. Referring to
both a property and a method has the same notation, and whether a property
will be accessed or a method will be called, solely depends on the context,
i.e. whether the usage is a variable access or a function call.

Example #7 Property access vs. method call

<?phpclass Foo{ public $bar = 'property';

public function bar() { return 'method'; }}

$obj = new Foo();echo $obj->bar, PHP_EOL, $obj->bar(), PHP_EOL;

The above example will output:

property
method

That means that calling an anonymous
function which has been assigned to a property is not directly
possible. Instead the property has to be assigned to a variable first, for
instance. As of PHP 7.0.0 it is possible to call such a property directly
by enclosing it in parentheses.

extends

A class can inherit the methods and properties of another class by
using the keyword extends in the class
declaration. It is not possible to extend multiple classes; a
class can only inherit from one base class.

The inherited methods and properties can be overridden by
redeclaring them with the same name defined in the parent
class. However, if the parent class has defined a method
as final, that method
may not be overridden. It is possible to access the overridden
methods or static properties by referencing them
with parent::.

When overriding methods, the parameter signature should remain the same or
PHP will generate an E_STRICT level error. This does
not apply to the constructor, which allows overriding with different
parameters.

::class

Since PHP 5.5, the class keyword is also used for class
name resolution. You can get a string containing the fully qualified name
of the ClassName class by using
ClassName::class. This is particularly useful with
namespaced classes.

Example #10 Class name resolution

<?phpnamespace NS { class ClassName { }

echo ClassName::class;}?>

The above example will output:

NS\ClassName

Note:

The class name resolution using ::class is a
compile time transformation. That means at the time the class name string
is created no autoloading has happened yet. As a consequence, class names
are expanded even if the class does not exist. No error is issued in
that case.

User Contributed Notes 18 notes

I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.

First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.

Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes.

What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.

$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.

But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.

Wouldn't it be great to get the lawn mowed by saying $son->mowLawn()? Assuming the function mowLawn() is defined, and you have a son that doesn't throw errors, the lawn will be mowed.

In the following example; let objects of type Line3D measure their own length in 3-dimensional space. Why should I or PHP have to provide another method from outside this class to calculate length, when the class itself holds all the neccessary data and has the education to make the calculation for itself?

<?php

/* * Point3D.php * * Represents one locaton or position in 3-dimensional space * using an (x, y, z) coordinate system. */class Point3D{ public $x; public $y; public $z; // the x coordinate of this Point.

My absolute favorite thing about OOP is that "good" objects keep themselves in check. I mean really, it's the exact same thing in reality... like, if you hire a plumber to fix your kitchen sink, wouldn't you expect him to figure out the best plan of attack? Wouldn't he dislike the fact that you want to control the whole job? Wouldn't you expect him to not give you additional problems? And for god's sake, it is too much to ask that he cleans up before he leaves?

I say, design your classes well, so they can do their jobs uninterrupted... who like bad news? And, if your classes and objects are well defined, educated, and have all the necessary data to work on (like the examples above do), you won't have to micro-manage the whole program from outside of the class. In other words... create an object, and LET IT RIP!

A PHP Class can be used for several things, but at the most basic level, you'll use classes to "organize and deal with like-minded data". Here's what I mean by "organizing like-minded data". First, start with unorganized data.

Imagination that each function you write only calls the bits of data in that class. Some functions may access all the data, while other functions may only access one piece of data. If each function revolves around the data inside, then you have created a good class.

$ob = new a();a::getFoo(); // output: I am foo $ob->getFoo(); // output: I am foo//a::getBar(); // fatal error: using $this not in object context$ob->getBar(); // output: I am bar // If you keep $bar non static this will work // but if bar was static, then var_dump($this->bar) will output null

// unset($ob);a::setFoo(); // The same effect as if you called $ob->setFoo(); because $foo is static$ob = new a(); // This will have no effects on $foo$ob->getFoo(); // output: I am a new foo

At first I was also confused by the assignment vs referencing but here's how I was finally able to get my head around it. This is another example which is somewhat similar to one of the comments but can be helpful to those who did not understand the first example. Imagine object instances as rooms where you can store and manipulate your properties and functions. The variable that contains the object simply holds 'a key' to this room and thus access to the object. When you assign this variable to another new variable, what you are doing is you're making a copy of the key and giving it to this new variable. That means these two variable now have access to the same 'room' (object) and can thus get in and manipulate the values. However, when you create a reference, what you doing is you're making the variables SHARE the same key. They both have access to the room. If one of the variable is given a new key, then the key that they are sharing is replaced and they now share a new different key. This does not affect the other variable with a copy of the old key...that variable still has access to the first room

Unless the function's parameter is marked as pass by reference using "&", arguments to it are passed by value. This goes for objects same as everything else. The fact is every object has its own unique identity, and the "value" of an object IS the object itself. And not, for example, anything to do with the values of any of its properties: modifying an object's properties doesn't change the object's identity (and hence its value) any more than your identity changes when you take your socks off. So when an object is passed (by value) to a function, and its properties are modified, THAT object's properties get modified.

The "&" modifier only matters if a new value is assigned to the parameter variable. If the modifier is used, then whatever was passed in the parameter is also replaced, if it's not, then it isn't. Again, this behaviour is the same whether it's an object or something else.